Catalytic processes are indispensable in the chemical industry. Frequently, catalytic processes employ a catalyst that is incorporated on a support. Effective use of the catalyst often corresponds to the quality of the catalyst support. Poor quality catalyst supports, due to at least one of physical degradation, chemical degradation, undesirable properties, and inconsistent properties, limit the effectiveness of catalysts incorporated therein. Conditions such as high temperatures, high pressures, and high or low pH environments present challenges to the integrity of catalyst supports.
For example, conventional catalyst composites for the purification of terephthalic acid by the Amoco mid-continent process (PTA catalysts) are composed of palladium-supported on granular 4×8 mesh carbon. These catalyst composites are designed to remove the two major impurities present in crude terephtahlic acid; namely yellow color and 4-carboxy benzaldehyde (4-CBA).
Carbon is the preferred support material for conventional PTA catalysts because it is essentially the only readily available material that can simultaneously yield an effective catalyst for color removal, 4-carboxy benzaldehyde removal, and also withstand the extremely corrosive environment of the terephthalic acid purification process. Although conventional carbon supported PTA catalysts have been used extensively over the past 20 years, such catalyst composites suffer from several disadvantages. These disadvantages include: highly irregular shapes leading to possible mal-distribution of liquid or gas flows in a catalytic reactor bed utilizing such catalyst composites; irregular shapes having sharp and fragile edges and corners which tend to break off and contaminate the PTA product with undesirable dust and black particles; brittleness which also leads to breakage and dust/black particles contaminating the PTA product; natural origin, i.e., coconut shell, which leads to non-uniformity form one growing season to another and consequent non-consistency of the carbon support; and being commonly derived from nutshells, such activated carbon is highly microporous, leading to the requirement of locating all of the active catalytic metal at the surface of the particles, where it is undesirably susceptible to loss during the movement and abrasion which occurs during shipping and handling.
Particularly problematic is the unpredictable and uncontrollable melange of irregular shapes and sizes associated with commonly employed granular cocoanut carbon supports. Granular cocoanut carbons are also mostly microporous; that is, they have numerous pores having a pore diameter less than 50 Å. As a result, the catalytic metals must be located near the exterior edges of the supports to avoid low activity due to mass transfer resistances. However, when catalytic metals are located near the exterior edges of supports, they are subject to loss due to mechanical attrition and thus the catalyst support loses its activity. Catalytic metals located near the exterior edges of a support are readily accessible to corrosion metals commonly present in reactor feeds and thus subject to deactivation.
Non-carbon catalyst supports are employed in catalytic processes in attempts to overcome the disadvantages associated with conventional carbon supported catalysts. Non-carbon supports include alumina supports, silica supports, alumina-silica supports, various clay supports, titania, and zirconium supports. However, there are at least one of two disadvantages associated with non-carbon catalyst supports; namely, that they may become weak and loose physical strength, that they are dissolved in highly corrosive environments (such as hot aqueous solutions of terephthalic acid) and that they have difficulties in removing undesirable color from crude terephthalic acid.
Improved catalyst supports and catalyst composites are therefore desired. Specifically, improved PTA catalyst supports and PTA catalyst composites are desired to provide improved methods of purifying terephthalic acid and improved useful lifetimes.